The present invention generally relates to game balls, and more particularly to game balls having covers containing crosslinkable thermoplastic polyurethane. The ball preferably is a molded game ball such as a golf ball, basketball, baseball, softball, football, soccer ball, volleyball, tennis ball, lacrosse ball or the like.
There are generally, three types of golf balls. The first type is a wound ball wherein a vulcanized rubber thread is wound under tension around a solid or semi-solid core, and thereafter is enclosed in a single or multi-layer covering of tough, protective material.
A second type of golf ball is a one-piece ball formed from a solid mass of moldable resilient material which has been cured to develop the necessary degree of hardness. One-piece molded balls do not have an enclosing cover.
A third type of ball is a multi-piece (two or more pieces) non-wound ball which includes a solid or liquid core of one or more layers and a cover having one or more layers formed over the core.
Multi-piece non-wound golf balls typically have a cover which contains a substantial quantity of ionomer. Useful ionomers include those sold by E. I. Dupont de Nemours and Company under the name Surlyn(copyright) as well as those sold by Exxon under the name Iotek(copyright). Ionomers impart toughness and cut resistance to the covers. It would be useful to develop golf ball covers which contain substantial quantities of non-ionomeric materials and which have the durability, scuff resistance, cut resistance and other playability properties of ionomeric golf ball covers.
Polyurethanes are polymers which are used to form a broad range of products. They are generally formed by mixing two primary ingredients during processing. For the most commonly used polyurethanes, the two primary ingredients are a polyisocyanate (for example, diphenylmethane diisocyanate monomer (xe2x80x9cMDIxe2x80x9d) and toluene diisocyanate (xe2x80x9cTDIxe2x80x9d) and their derivatives) and a polyol (for example, a polyester polyol or a polyether polyol).
A wide range of combinations of polyisocyanates and polyols, as well as other ingredients, are available. Furthermore, the end-use properties of polyurethanes can be controlled by the type of polyurethane utilized, i.e., whether the material is thermoset (crosslinked molecular structure) or thermoplastic (linear molecular structure).
Crosslinking occurs between the isocyanate groups (xe2x80x94NCO) and the polyol""s hydroxyl end-groups (xe2x80x94OH). Additionally, the end-use characteristics of polyurethanes can also be controlled by different types of reactive chemicals and processing parameters. For example, catalysts are utilized to control polymerization rates. Depending upon the processing method, reaction rates can be very quick (as in the case for some reaction injection molding systemsxe2x80x94xe2x80x9cRIMxe2x80x9d) or may be on the order of several hours or longer (as in several coating systems). Consequently, a great variety of polyurethanes are suitable for different end-uses.
Polyurethane has been used for golf balls and other game balls as a cover material. Commercially available polyurethane golf balls have been made of thermoset polyurethanes. A polyurethane becomes irreversibly xe2x80x9csetxe2x80x9d when a polyurethane prepolymer is crosslinked with a polyfunctional curing agent, such as polyamine and polyol. The prepolymer typically is made from polyether or polyester. Diisocyanate polyethers are preferred because of their water resistance.
The physical properties of thermoset polyurethanes are controlled substantially by the degree of crosslinking. Tightly crosslinked polyurethanes are fairly rigid and strong. A lower amount of crosslinking results in materials that are flexible and resilient. Thermoplastic polyurethanes have some crosslinking, but purely by physical means. The crosslinking bonds can be reversibly broken by increasing temperature, as occurs during molding or extrusion. In this regard, thermoplastic polyurethanes can be injection molded, and extruded as sheet and blown film. They can be used to up to about 350xc2x0 F. and are available in a wide range of hardnesses.
U.S. Pat. No. 5,006,297 indicates that while thermoplastic and thermosetting polyurethanes are known, thermosets have been found to produce better cover stocks for golf balls. Additionally, while thermoplastic polyurethanes can be used to form game balls, they lack the scuff and cut resistance of a crosslinked polyurethane. Similarly, thermoplastic polyurethanes do not readily crosslink.
A further disadvantage of using thermosetting polyurethanes to form game ball covers is that scrap material (i.e. sprues, runners and/or reject parts) and cover stock from off-spec balls cannot be reused without substantial processing. It would be useful to develop a high quality game ball utilizing a polyurethane cover material which is subject to thermal degradation prior to final processing. In such a case, the scraps formed in the cover molding stage could be conveniently re-used to form additional game ball covers. A further advantage would be to produce a polyurethane based game ball which, when molded and then crosslinked, is resistant to thermal degradation. This would produce an improved game ball which could also withstand prolonged exposure to heat during use or storage.
The present invention relates to new and improved game balls which overcome the above-referenced problems and others. An object of the invention is to form a durable, scuff resistant game ball. The invention includes unitary, wound, two-piece, three-piece and multi-layer golf balls, but is not limited solely to golf balls.
Another object of the invention is to provide a game ball cover in which scrap cover material can be readily reused prior to final processing.
Yet another object of the invention is to provide a golf ball having a scuff resistant polyurethane cover which is also resistant to heat elongation at high temperatures.
Yet another object of the invention is to provide an improved method for making a thermoplastic polyurethane covered game ball
Yet another object of the invention is to provide a method for making a scuff resistant and cut resistant polyurethane game ball.
Another object of the invention is to provide a method of making a polyurethane covered golf ball having high heat resistance.
An additional object is to produce a thermoplastic polyurethane game ball which is readily crosslinked via high energy electrons or gamma rays. Such a thermoplastic polyurethane would be easily processable and could be directly molded into or around a core to form a game ball. Alternatively, the thermoplastic polyurethane could be injection molded into half shells that could be compression molded around a core or a mantle to form a multi-piece game ball. The runners and scraps from the molding process would not be crosslinked and could then be recycled with the virgin cover material. After molding, the game ball could then be subjected to electron beam or gamma irradiation.
Other objects will be pointed out more particularly in detail hereafter.
The present invention addresses and remedies all of the foregoing objectives. In a first aspect, the present invention provides a game ball comprising a central portion and a first cover layer formed over the central portion. The first cover layer is formed from a particular type of crosslinkable thermoplastic polyurethane.
In another aspect, the present invention provides a core and a cover layer disposed about the core. The cover layer comprises a thermoplastic polyurethane that is capable of undergoing crosslinking upon exposure to about 3.5 Mrads of radiation, thereby causing an increase in the hardness of the cover by at least 2 units on the Shore D hardness scale.
In yet another aspect, the present invention provides a method of forming a game ball comprising providing a game ball center and then forming a cover layer over the game ball center. The cover layer includes a crosslinkable thermoplastic polyurethane.
In a further aspect, the present invention provides a method of making a golf ball comprising providing a core of a particular composition, forming a cover layer about the core, and then irradiating the cover layer under conditions sufficient to increase the Shore D hardness of the cover layer by at least 3 units. The cover layer comprises a resin composition that includes at least 95 parts by weight of a crosslinkable thermoplastic polyurethane and has a hardness prior to crosslinking, of 35 to about 72 on the Shore D hardness range.